Physicists Get One Step Closer to Discovering Dark Photons in the Universe

In the last few years of its functioning, a particle collider located in Northern California was refocused to look for the existence of new particles that may assist in filling the gap in our knowledge of the universe.

The BaBar detector at SLAC National Accelerator Laboratory. CREDIT: SLAC.

A fresh examination of the data co-headed by physicists from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), restricts certain hiding places for one kind of theorized particle, the dark photon (or the heavy photon), which was hypothesized to assist in elucidating the mystery behind dark matter.

The outcomes of the study have been reported in the Physical Review Letters journal by the approximately 240-member BaBar Collaboration. These results complement the outcomes of a collection of earlier experiments searching for the theorized dark photons.

“Although it does not rule out the existence of dark photons, the BaBar results do limit where they can hide, and definitively rule out their explanation for another intriguing mystery associated with the property of the subatomic particle known as the muon,” stated Michael Roney, a University of Victoria professor and BaBar spokesperson.

Approximately 85% of the total mass of the universe is formed of dark matter. However, dark matter has been noticed only through its gravitational interactions with normal matter. For instance, the rate of rotation of the galaxies is considerably faster than anticipated dependant upon the visible matter, indicating that there is “missing” mass that has been invisible to researchers thus far.

Therefore, physicists have spent considerable time on theories and experiments to account for the composition of dark matter, for instance, whether it is formed of unexplored particles, and whether a hidden or “dark” force controls the interactions of these particles both within themselves and with visible matter. The dark photon, on the condition that it exists, has been proposed to be a prospective bearer of this dark force.

The researchers used data collected from SLAC National Accelerator Laboratory in Menlo Park, California, during the period 2006-2008 and scanned the recorded secondary products of particle collisions for indications of a single light particle (or a photon) lacking associated particle processes.

The BaBar experiment carried out at SLAC from 1999 to 2008 collected data related to collisions of electrons with positrons, or positively charged antiparticles of electrons. PEP-II, the collider that drives BaBar, was developed as part of a partnership among SLAC, Berkeley Lab, and Lawrence Livermore National Laboratory. During the peak years of the BaBar Collaboration, more than 630 physicists from 13 countries were involved in it.

BaBar was initially developed for analyzing the differences in the interactions between matter and antimatter forming a b-quark. At the same time as a competing experiment known as Belle performed in Japan, BaBar proved the hypotheses of theorists and opened the door for the 2008 Nobel Prize. Pier Oddone, a physicist from Berkeley Lab, put forward the concept for BaBar and Belle in 1987 when he was working as the Physics division director of the Lab.

The fresh analysis adopted nearly 10% of data from BaBar, recorded in the last 2 years of functioning of BaBar. Data collection at BaBar was refocused to find particles not included in the Standard Model of physics, which is a kind of rulebook for the type of particles and forces forming the known universe.

“BaBar performed an extensive campaign searching for dark sector particles, and this result will further constrain their existence,” stated Bertrand Echenard, a research professor at Caltech who played a vital role in this attempt.

The signature (of a dark photon) in the detector would be extremely simple: one high-energy photon, without any other activity.

Related Stories

Many dark photon theories have proposed that the related dark matter particles will be invisible to the detector. The single photon emitted from a beam particle indicates that there occurred an electron-positron collision and that the invisible dark photon was disintegrated into the dark matter particles, disclosing itself even without any accompanying energy.

At the time physicists had predicted the existence of dark photons in the year 2009, it kindled the interest of all the physicists, and induced a fresh exploration of BaBar data. Kolomensky oversaw the data analysis conducted by Mark Derdzinski and Alexander Giuffrida, undergraduates at UC Berkeley.

“Dark photons could bridge this hidden divide between dark matter and our world, so it would be exciting if we had seen it,” stated Kolomensky.

It has also been hypothesized that the dark photon can account for the discrepancy between the observation of a specific characteristic of the muon spin and the value proposed for the same in the Standard Model. Measurement of this characteristic with unmatched accuracy is the aim of the Muon g-2 (pronounced gee-minus-two) Experiment at Fermi National Accelerator Laboratory.

Measurements conducted earlier at Brookhaven National Laboratory showed that this characteristic of muons (similar to a wobble in a spinning top, which is slightly off the norm) deviated by nearly 0.0002% from the anticipated value. Dark photons were proposed to be the probable particle to solve this mystery, and a fresh round of studies which began at the start of this year should be helpful in deciding whether the anomaly is really a novel finding.

According to Kolomensky, the fresh BaBar outcome principally “rules out these dark photon theories as an explanation for the g-2 anomaly, effectively closing this particular window, but it also means there is something else driving the g-2 anomaly if it’s a real effect.”

It is a constant and usual reciprocity between theory and experiments, wherein theory adjusts to new constraints thrown by experiments, and experiments look for stimulation from innovative and adjusted theories to discover the next proving grounds for investigating these theories.

According to Roney, researchers have been frantically investigating data collected from BaBar to take exploit well-perceived experimental conditions and detector to investigate new theoretical ideas.

“Finding an explanation for dark matter is one of the most important challenges in physics today, and looking for dark photons was a natural way for BaBar to contribute,” stated Roney said, further noting that a number of experiments being carried out at present or planned globally are trying to overcome this challenge.

An advancement to an experiment conducted in Japan, which is identical to BaBar (known as Belle II) is to start next year.

Eventually, Belle II will produce 100 times more statistics compared to BaBar, experiments like this can probe new theories and more states, effectively opening new possibilities for additional tests and measurements.

“Until Belle II has accumulated significant amounts of data, BaBar will continue for the next several years to yield new impactful results like this one,” stated Roney.

The research involved participation of the international BaBar collaboration, including scientists from nearly 66 institutions in the United States, Canada, France, Spain, Italy, Norway, Germany, Russia, India, Saudi Arabia, the United Kingdom, the Netherlands, and Israel. The U.S. Department of Energy’s Office of Science and the National Science Foundation, the Natural Sciences and Engineering Research Council in Canada; CEA and CNRS-IN2P3 in France, BMBF and DFG in Germany, INFN in Italy, FOM in the Netherlands, NFR in Norway, MES in Russia, MINECO in Spain, STFC in the United Kingdom, and BSF in Israel and the United States supported the research. The Marie Curie EIF in the European Union and the Alfred P. Sloan Foundation in the United States supported the researchers who were part of this study.